1. Field of the Invention
The present invention relates to metallic articles having vitreous or glass-ceramic pigmented protective layers exhibiting a high chemical resistance.
2. Discussion of Background Information
Vitreous or glass-ceramic protective layers are applied to metallic surfaces generally by what is called an enameling operation. Enamels are glasses which melt relatively readily at relatively high temperatures and then form a coherent coating on the metal surface. The melting temperatures are generally between 750 and 800° C. when enamels with a sufficient chemical resistance are to be achieved, for example toward boiling water, weak acids or alkaline, boiling or cooking foods, etc. The latter is required particularly in applications in the foods sector.
The enamels are applied in a wet process, wherein an aqueous slurry or suspension (engobe) of the finely ground glass powder (frit) is generally applied to the metal surface (for example by dipping or spray application), dried and then melted at the abovementioned temperatures. To achieve a coherent, impervious, pore-free coating, layer thicknesses of 50 to 100 μm are required. Owing to the high viscosity of glasses and the high softening temperatures which exist particularly in the case of silicate glasses, as already mentioned, melting temperatures of well above 700° C. are required. The softening temperature depends to a particularly high degree on the silica content of the glass. High silica contents lead to a high softening temperature and result in a high chemical resistance of the glass. High alkali metal contents, in contrast, result in a low softening temperature but lead to a low chemical resistance (low hydrolytic class). Especially for the use of such layers in the field of aggressive media (for example acidic) or in the foods sector, especially when, for example, machine dishwasher stability is required, the low-melting layers mentioned are therefore unsuitable. This is also one of the reasons why, for example, enamel is not used on aluminum in the domestic appliance sector, especially in cookware, because aluminum already melts at temperatures a little above 600° C. The situation is analogous in the case of magnesium or in the case of magnesium-aluminum alloys. The same also applies to metal assemblies composed of several components when one of these components is from the range of the abovementioned lightweight metals.
Furthermore, coloring is of crucial importance for many applications since the question of an attractive design is of high significance for many articles, particularly in the consumer goods sector. In the foods sector, the question of food compatibility and of toxicology in particular is a predominant factor. For this reason, the chemical resistance of a vitreous coating is of particular significance especially when it is necessary to prevent leaching of components, for example metal ions, out of coloring pigments.
It was therefore an object of the invention to provide metallic substrates with a colored vitreous coating having improved chemical properties, especially improved alkali stability extending as far as machine dishwasher stability.
Important prerequisites for the achievement of corrosion-resistant layers are a high stability of the matrix material of the coating, especially toward alkalis, crack- and pinhole-free consolidation, for which an appropriately matched coefficient of expansion of the coating material to the substrate is also required. However, it is known that, in the case of pigmented systems in which shrinkage of the coating matrix is unavoidable, nonshrinking pigments, for example oxidic pigments, lead to stresses and cracking in the course of sintering. It is known from the relevant glass literature that the alkali stability of silicate glasses can be improved significantly by addition of particular ions (network-stabilizing components). Possible solutions to the problem are therefore glass compositions which comprise such components (for example aluminum oxide or titanium dioxide). As likewise known from the sol-gel literature, sol-gel coatings with such multicomponent systems are difficult to produce, have exceptionally short pot life, are generally stable only in strongly acidic solutions and are therefore barely practicable in industry. If such systems are to be produced with high coefficients of expansion in order to be matched to the coefficients of expansion of metals (TCE≈8×10−6/K or greater), the alkali metal contents must be increased well above the level of conventional glasses (10 to 15 percent by weight), which is in turn associated with a severe loss of chemical stability.
Patent specifications U.S. Pat. No. 6,162,498 and US 2008/0118745 describe processes in which vitreous, relatively abrasion-resistant layers resistant to oxidation corrosion (for example tarnishing of stainless steel) are obtained. The process comprises                the production of a coating solution via hydrolysis and polycondensation of one or more silanes in the presence of colloidal silica sol and of at least one component from the group of the alkali metal and alkaline earth metal oxides and hydroxides;        the application of the coating solution to a metal surface to form a layer;        thermal densification to form a vitreous film;        in U.S. Pat. No. 6,162,498, the use of densification temperatures between 350 and 500° C.;        in US 2008/0118745, additionally the formation of formable vitreous layers, via        the use of an alkali metal silicate-containing layer through densification in a two-stage operation with a preferred temperature of 500° C.;        the production of layer systems via dipping and spraying with layers in the range from 5 to 10 μm;        the application of the coatings to metal surfaces and metal components, especially to stainless steel, but also to aluminum and aluminum alloys.        
The systems described have a high SiO2 content and are therefore stable only to acids, and not in an alkaline medium. Thus, these layers can be removed quantitatively even with relatively dilute hot sodium hydroxide solution and are not machine dishwasher-stable in any case, which means that they are not normally suitable for the foods sector or as protective layers for applications at relatively high pH values.
However, these layers are also suitable for application to aluminum and aluminum alloys since the densification temperatures thereof are well below 600° C.; however, this does not improve chemical stability with regard to alkali resistance. The layers are also suitable for coating of components and assemblies composed of aluminum or aluminum alloys and other metals, whether in the form of a laminate formed from metal sheets or plates, in the form of a sandwich, or assemblies comprising components made from different metals which are bonded or joined to one another in some other way (for example screwed, pressed or riveted).
It has now been found that, surprisingly, the disadvantages outlined above, especially the insufficient chemical stability of such coating materials, are alleviated or avoided when they are used in conjunction with pigments, preferably platelet-shaped pigments, but especially when they are applied as a multiple layer. They then have a distinct increase in hydrolytic stability and, in the machine dishwasher test, withstand several hundred cycles without impairment.
In addition, it was also surprising that the pigmented coating solutions achieve layers which sinter to produce impervious layers, even though sintering operations in which thermal densification of the matrix material inevitably leads to shrinkage, with oxidic additives (i.e. already imperviously sintered additives which by their nature cannot shrink any further), generally cause internal crack formation. The latter worsens the chemical stability in particular to an exceptional degree since corrosive liquids penetrate into the layer through the cracks and cause corrosion processes at the metal surface resulting in detachment phenomena. In addition, it was also surprising that the layer systems for which an estimate of the coefficients of expansion of the matrix gave values between α=1.0 and 4.0×10−6/K due to their composition, in spite of differences between the coated metals (α>10), did not give rise to any deterioration in the imperviosity of the layer. Coating solutions with compositions corresponding to customary soda-lime glasses with corresponding thermal properties were produced in the context of the invention, but are not preferred due to the problems described above. Moreover, the coefficients of expansion which are measured in solid glass are only of minor relevance for thin layers since structures which arise in thin layers differ significantly from the structure of solid glass and hence also have different properties.